This invention relates to a circuit for adjusting to the sensitivity of a sensor, of the type which generally is used in various paper or document handling equipment, for example, and it also relates to a special application of the circuit in which a plurality of such sensors is used in a height-determining operation in conjunction with an imaging device.
The most common type of "sensor" used currently in document-handling equipment consists of an optically-coupled, light-emitting diode/phototransistor pair. When the "pairs" are "unselected", i.e., selected at random, two such pairs can exhibit a sensitivity range of about two orders of magnitude or more. A common solution to this problem is to match the light emitting diode (LED) and the associated phototransistor or photodetector in pairs in order to achieve a desired sensitivity range. Final adjustment of the sensitivity is done by adjusting the current to the LED of an associated pair. This adjustment increases the cost of the sensor and complicates its maintenance because the LED and its associated phototransistor must be replaced in "pairs".
Another problem with sensors used in document-handling equipment is that the light-transmitting qualities of the associated documents may range from being near opaque to being almost transparent. Naturally, this limits the maximum sensitivity of the detector in the "pair" that can be used in an application.
Dust accumulating on the sensor is a third problem which is common with the equipment discussed. The accumulation of dust can eventually reach a level at which the dust cannot be distinguished from a document, and consequently, the sensor becomes useless.
Also, the sensitivity of the photodetector changes with age.